Sphingomyelin (d18:0/16:1(9Z)) or SM(d18:0/16:1(9Z)) is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath which surrounds some nerve cell axons. It usually consists of phosphorylcholine and ceramide. In humans, sphingomyelin is the only membrane phospholipid not derived from glycerol. Like all sphingolipids, SPH has a ceramide core (sphingosine bonded to a fatty acid via an amide linkage). In addition it contains one polar head group, which is either phosphocholine or phosphoethanolamine. The plasma membrane of cells is highly enriched in sphingomyelin and is considered largely to be found in the exoplasmic leaflet of the cell membrane. However, there is some evidence that there may also be a sphingomyelin pool in the inner leaflet of the membrane. Moreover, neutral sphingomyelinase-2 - an enzyme that breaks down sphingomyelin into ceramide has been found to localise exclusively to the inner leaflet further suggesting that there may be sphingomyelin present there. Sphingomyelin can accumulate in a rare hereditary disease called Niemann-Pick Disease, types A and B. Niemann-Pick disease is a genetically-inherited disease caused by a deficiency in the enzyme Sphingomyelinase, which causes the accumulation of Sphingomyelin in spleen, liver, lungs, bone marrow, and the brain, causing irreversible neurological damage. SMs play a role in signal transduction. Sphingomyelins are synthesized by the transfer of phosphorylcholine from phosphatidylcholine to a ceramide in a reaction catalyzed by sphingomyelin synthase.

This compound belongs to the class of organic compounds known as phosphosphingolipids. These are sphingolipids with a structure based on a sphingoid base that is attached to a phosphate head group. They differ from phosphonospingolipids which have a phosphonate head group.

Bidirectional lipid cholinephosphotransferase capable of converting phosphatidylcholine (PC) and ceramide to sphingomyelin (SM) and diacylglycerol (DAG) and vice versa. Direction is dependent on the relative concentrations of DAG and ceramide as phosphocholine acceptors. Directly and specifically recognizes the choline head group on the substrate. Also requires two fatty chains on the choline-P donor molecule in order to be recognized efficiently as a substrate. Does not function strictly as a SM synthase. Required for cell growth

Bidirectional lipid cholinephosphotransferase capable of converting phosphatidylcholine (PC) and ceramide to sphingomyelin (SM) and diacylglycerol (DAG) and vice versa. Direction is dependent on the relative concentrations of DAG and ceramide as phosphocholine acceptors. Directly and specifically recognizes the choline head group on the substrate. Also requires two fatty chains on the choline-P donor molecule in order to be recognized efficiently as a substrate. Does not function strictly as a SM synthase. Suppresses BAX-mediated apoptosis and also prevents cell death in response to stimuli such as hydrogen peroxide, osmotic stress, elevated temperature and exogenously supplied sphingolipids. May protect against cell death by reversing the stress-inducible increase in levels of proapoptotic ceramide. Required for cell growth

Involved in the biosynthesis of L2/HNK-1 carbohydrate epitope on glycoproteins. Can also play a role in glycosaminoglycan biosynthesis. Substrates include asialo-orosomucoid (ASOR), asialo-fetuin, and asialo-neural cell adhesion molecule. Requires sphingomyelin for activity: stearoyl-sphingomyelin was the most effective, followed by palmitoyl-sphingomyelin and lignoceroyl-sphingomyelin. Activity was demonstrated only for sphingomyelin with a saturated fatty acid and not for that with an unsaturated fatty acid, regardless of the length of the acyl group (By similarity).